Use strong zeros in the diffrules for min/max (#87)

Author: devmotion

src/rules.jl

@@ -90,8 +90,8 @@ _abs_deriv(x) = signbit(x) ? -one(x) : one(x)
 @define_diffrule Base.mod(x, y)    = :( z = $x / $y; ifelse(isinteger(z), oftype(float(z), NaN), one(float(z))) ), :(  z = $x / $y; ifelse(isinteger(z), oftype(float(z), NaN), -floor(float(z))) )
 @define_diffrule Base.rem(x, y)    = :( z = $x / $y; ifelse(isinteger(z), oftype(float(z), NaN), one(float(z))) ), :(  z = $x / $y; ifelse(isinteger(z), oftype(float(z), NaN), -trunc(float(z))) )
 @define_diffrule Base.rem2pi(x, r) = :( 1                                                       ), :NaN
-@define_diffrule Base.max(x, y)    = :( $x > $y ? one($x) : zero($x)                            ), :( $x > $y ? zero($y) : one($y)                                            )
-@define_diffrule Base.min(x, y)    = :( $x > $y ? zero($x) : one($x)                            ), :( $x > $y ? one($y) : zero($y)                                            )
+@define_diffrule Base.max(x, y)    = :( $x > $y ), :( !($x > $y) )
+@define_diffrule Base.min(x, y)    = :( !($x > $y) ), :( $x > $y )
 
 # trinary #
 #---------#